Cancer immunotherapy involves the use of a subject's own immune system to treat or prevent cancer. Immunotherapies exploit the fact that cancer cells often have subtly different molecules on their surface that can be detected by the immune system. These molecules, or cancer antigens, are most commonly proteins, but also include molecules such as carbohydrates. Immunotherapy thus involves provocation of the immune system into attacking tumour cells via these target antigens. However, malignant tumours, in particular solid tumours, can escape immune surveillance by means of various mechanisms both intrinsic to the tumour cell and mediated by components of the tumour microenvironment. Amongst the latter, tumour infiltration by regulatory T cells (Treg cells or Tregs) and, more specifically, an unfavourable balance of effector T cells (Teff) versus Tregs (i.e. a low ratio of Teff to Treg), have been proposed as critical factors (Smyth M et al., 2014).
Since their discovery, Tregs have been found to be critical in mediating immune homeostasis and promoting the establishment and maintenance of peripheral tolerance. However, in the context of cancer their role is more complex. As cancer cells express both self- and tumour-associated antigens, the presence of Tregs, which seek to dampen effector cell responses, can contribute to tumour progression. The infiltration of Tregs in established tumours therefore represents one of the main obstacles to effective anti-tumour responses and to treatment of cancers in general. Suppression mechanisms employed by Tregs are thought to contribute significantly to the limitation or even failure of current therapies, in particular immunotherapies that rely on induction or potentiation of anti-tumour responses (Onishi H et al; 2012).
It has been consistently demonstrated that Treg cells contribute to the establishment and progression of tumors in murine models and that their absence results in delay of tumor progression (Elpek et al., 2007; Golgher et al., 2002; Jones et al., 2002; Onizuka et al., 1999; Shimizu et al., 1999). In humans, high tumor infiltration by Treg cells and, more importantly, a low ratio of effector T (Teff) cells to Treg cells, is associated with poor outcomes in multiple cancers (Shang et al., 2015). Conversely, a high Teff/Treg cell ratio is associated with favourable responses to immunotherapy in both humans and mice (Hodi et al., 2008; Quezada et al., 2006). Nevertheless, depletion of Tregs in tumours is complex, and results of preclinical and clinical studies in this area had been inconsistent, mostly due to the difficulty of identifying a target specific for Treg.
CD25 is one of the potential molecular targets for achieving depletion of Tregs. Indeed, CD25, also known as the interleukin-2 high-affinity receptor alpha chain (IL-2Rα), is constitutively expressed at high-levels on Treg cells, and it is absent or expressed at low-levels on T effector cells and is thus a promising target for Treg depletion. The IL-2/CD25 interaction has been the object of several studies in murine models, most of them involving the use of PC61, a rat anti-murine CD25 antibody (Setiady Y et al., 2010.). The CD25 binding and functional activities of this antibody have been compared to those of panel of monoclonal antibodies generated by different authors (Lowenthal J. W et al., 1985; Moreau, J.-L et al.; Volk H D et al., 1989; Dantal J et al., 1991). While original studies demonstrated prophylactic but not therapeutic activity of PC61, a recent study showed that an Fc optimized version of this anti-CD25 antibody led to intra-tumoral Treg depletion and offers significant therapeutic benefit in several murine tumour models (Vargas A et al., 2017).
Available anti-CD25 antibodies such as PC61 block or inhibit the binding of IL-2 to CD25, as do many other anti-mouse CD25 antibodies and most of the antibodies disclosed as being anti-human CD25 antibodies; see for instance WO2004/045512, WO2006/108670, WO1993/011238, WO1990/007861 and WO2017/174331. For example, basiliximab and daclizumab are anti-human CD25 antibodies that inhibit the binding of IL-2 to CD25 and have been developed to reduce activation of T-effector cells (Queen C et al, 1989 and Bielekova B, 2013). Basiliximab is a chimeric mouse-human CD25 antibody currently approved for graft versus host diseases and daclizumab is a humanized CD25 antibody approved for the treatment of multiple sclerosis.
A few other anti-CD25 antibodies still allow the binding of IL-2 to CD25, such as the clone 7D4 (anti-mouse CD25), clone 2E4 (anti-mouse CD25), clone MA251 (anti-human CD25) or 7G7B6 (anti-human CD25) (i.e. non-blocking antibodies). Inolimomab/BT536, whilst it is been purported not to block binding of IL-2 to CD25, does block the signalling of IL-2 via CD25. 7D4 is a rat IgM anti-mouse CD25 antibody that has been extensively used to detect CD25-positive cells in the presence of or following the treatment with PC61 or of antibodies having similar binding properties (Onizuka S et al., 1999). Very few documents disclose any functional property of 7D4-IgM antibody, alone or in comparison with PC61 (Kohm A et al., 2006; Hallett W et al., 2008; Fecci P et al., 2006; McNeill A et al., 2007; Setiady Y et al., 2010; Couper K et al., 2007). Indeed, the prior art does not teach the possibility to adapt or somehow modify the isotype or other structural features of 7D4 in order to obtain an improved antibody, in particular those that can be used in cancer therapy. The ability of 7D4-IgM (as such or as an engineered antibody) or of any anti-human CD25 designed or characterized as having CD25 binding features similar to those of 7D4 for mouse CD25 have not been evaluated in detail with respect to the optimized depletion of Treg cells.
As discussed above the infiltration of Treg cells in tumors, and in particular a low ratio of effector Teff cells to Treg cells, can lead to poor clinical outcome. CD25 has been identified as a Treg marker and could thus be an interesting target for therapeutic antibodies aiming at depleting Treg. Importantly, CD25 is the alpha subunit of the receptor for IL-2 and IL-2 is a key cytokine for Teff responses. Anti-CD25 antibodies that have undergone clinical testing so far whilst depleting Treg cells also block IL-2 signalling via CD25 (specifically a CD25/CD122/CD132 complex). The present inventors have now found that such a blockade of IL-2 signalling limits Teff responses and that an anti-CD25 antibody that does not block the IL2 signalling can effectively deplete Treg cells, whilst still allowing IL-2 to stimulate Teff cells, providing antibodies that exhibit a strong anti-cancer effect.
Thus, there is a need in the art for improved anti-CD25 antibodies, in particular those that do not block the binding of CD25 to IL-2 or IL-2 signalling, and that deplete Tregs, in particular in tumours, and that can be used in methods for treating cancer.